The field of the present invention is polyamide powders for the coating of metals.
The invention is particularly concerned with methods for the production of pulverulent coating compositions based on polyamides having at least ten aliphatically bound carbon atoms per carbonamide group by the precipitation method. Copolyamides or a mixture of homo- and copolyamides containing at least 70% of the stated components are also useful.
The state of the prior art of producing polyamide powders may be ascertained by reference to U.S. Pat. Nos. 3,476,711; 3,900,607; 3,927,141; 3,966,838; 4,143,025; 4,195,162; and 4,334,056; British Pat. Nos. 535,138 and 688,771; and the Kirk-Othmer "Encyclopedia of Chemical Technology", 2nd Ed., Vol. 16(1968), under the section "Polyamide (Plastics)", pages 88-105, particularly page 92-polylauryllactam (nylon-12), and polyundecanamide (nylon-11), page 101 Solution Processes, and Powder Processing, pages 101-102, the disclosures of which are incorporated herein by reference.
The flame spraying and fluidized bed coating of nylon on a metal base is disclosed in U.S. Pat. No. 3,203,822.
It is known to make use of polyamide based coating compositions to prepare varnish-type coatings on metals. The coating takes place by the melt-film coating method, namely fluidized bed, flame-spraying or electrostatic coating. The polyamide powders are prepared by precipitating the polyamide from solutions as disclosed in British Pat. No. 688,771, or by grinding the polyamide granulate, preferably at low temperatures and in an inert atmosphere.
The precipitation method disclosed in British Pat. No. 688,771 is carried out by the precipitation of polyamide powders from ethanol by cooling the hot polyamide solution by external means or by letting it stand to cool. Powders with a high proportion of fine grains causing dust ejection while in the fluidized bed are obtained solely by cooling or letting stand to cool the hot polyamide solution. Lastly such a procedure also entails undesired agglomerates which subsequently require extra steps of being ground or abraded into the necessary powder size. This precipitation procedure furthermore delivers product batches which are unreproducible, that is, which are different in range of particle size, molecular weight and bulk density.
It is further known to prepare polyamide powders by grinding polyamides of low molecular weights and then raising them to the proper viscosity by heating them to temperatures below the melting point as disclosed in British Pat. No. 535,138 and U.S. Pat. No. 3,476,711.
Again powders of polylauryllactam are prepared by this known method and are used for coating in the manner as disclosed in Chem. Ind., November 1968, pp 783-791, and Modern Plastics, February 1966, pp 153-156. Because polylauryllactam powders do not always meet the requirements of high elasticity, good edge coating, smooth surface, resistance against alkaline aqueous solutions, and because during their processing they frequently tend in particular to emit thick smoke, the most diverse improvements have become known, illustratively polylauryllactam powders containing plasticizers as disclosed in U.S. Pat. No. 3,900,607, or those made of a mixture of homopolylauryllactam and copolyamides containing lauryllactum as disclosed in British Pat. No. 1,392,949, or those containing polyamides with N-alkoxymethyl groups besides acidically reacting catalysts as disclosed in U.S. Pat. No. 3,966,838, or mixtures of polyamides having 8 to 11 aliphatically bound carbon atoms per carbonamide group, aminoplasts bearing alkoxyalkyl groups and acidically reacting catalysts as disclosed in U.S. Pat. No. 3,927,141. While in particular cases these powders evince good properties, they nevertheless fail to meet all the required conditions.
U.S. Pat. Nos. 4,143,025 and 4,195,162 describe an improved process. However, this process still is not entirely satisfactory because a grinding procedure is employed to prepare pigment-free powders and because the precipitation method must be used to prepare pigmented powders. Lastly, the success of this process presumes the use of polylauryllactam granulates which were produced solely by hydrolytic polymerization in the presence of specific amounts of phosphoric acid.
U.S. Pat. No. 4,334,056 describes a further substantially improved process comprising a method of producing polyamide powder from polyamide having a relative viscosity of 1.4 to 1.8 as measured in 0.5% meta-cresol solution at 25.degree. C. and at least 70% of said polyamide consisting of polyamide with at least 10 aliphatically bound carbon atoms per carbonamide group, comprising:
(a) dissolving said polyamide in at least twice the amount by weight of ethanol in a closed vessel at a temperature between about 130.degree. to 150.degree. C. to form a solution of said polyamide; PA1 (b) cooling said solution to a precipitation temperature between about 100.degree. and 125.degree. C. and ceasing said cooling at said precipitation temperature; PA1 (c) precipitating said polyamide powder from said cooled solution of (b) polytropically with agitation and under an inert gas atmosphere; and PA1 (d) separating said preciptated polyamide powders of (c) from said ethanol.
Still this process incurs the drawback of relatively long precipitation times and of mechanical properties all of which leave room for improvement.